Efforts began several years ago to develop a cardiac pacemaker energy source (Plutonium 238) suitable for human implantation. Since May 1969, sixty-five atomic powered pacemakers have been implanted in dogs for in vivo testing; of these, 30 are the currently used NU-5 series. The first human implant was done April 9, 1973 and the second implant in June of 1973. Thirty dogs were implanted with NU-5 series between July and September 1972.